XIX IMEKO World Congress
Fundamental and Applied Metrology
September 6−11, 2009, Lisbon, Portugal
1
SAMEER- Centre for Electromagnetics, Chennai - 600113, India.
2
Institute of Electrical Measurement and Measurement Signal Processing,
Graz University of Technology, Graz-8010, Austria, bgeorge@tugraz.at.
3
Dept. of Electrical Engineering, Indian Institute of Technology Madras, Chennai - 600036, India.
– A virtual instrumentation based scheme is
developed for the characterization of magnetic materials.
The principle used for measurement is derived from the
comparison method of testing instrument transformers. A
prototype has been developed by suitably modifying an
instrument transformer test set. Characterization of magnetic
materials at single frequency and multiple frequencies have
been carried out. Typical test results obtained are presented
in the paper. The developed instrument substantially reduces
the testing time associated with magnetic characterization.
: Instrument transformers, magnetic
measurement, watt-loss measurement.
One of the largest users of magnetic materials in the
form of laminations and toroidal cores is the electric power
sector. Power transformers almost always use Si-steel
laminations. Current transformers (CTs) are unique in that
the magnetic structure is usually of a toroidal form.
Depending on the application, CTs use high permeability
Ni – Fe or Si – steel cores. Often, composite cores [1] are
employed. Designers of CTs require the magnetic
characteristics of core in the form of watt-loss and
magnetizing ampere turns as a function of the flux density.
Instruments with both sinusoidal B and H [2], [3] are
available for performing such measurements. In the recent
years, the advent of significant power electronic switching
devices has meant that the voltage and current waveforms in
the power network are not of a single frequency and contain
significant harmonics. An instrument transformer test set for
meeting this need has been designed [4]. It then becomes
necessary to characterize the magnetic materials as a
function of frequency. We develop a Virtual Instrumentation
(VI) based scheme for this purpose. The new scheme uses a
modified Instrument Transformer Test Set (ITTS) for the
characterization of magnetic materials. The proposed
method also shows a new application for the ITTS, with
certain modifications, as an instrument for the
characterization of magnetic materials.
! "" #
#
The Ratio Error (RE) and Phase Error (PE) in a CT are
principally due to the excitation characteristic of the core
and it is known [5] that
+
− =
2
cos sin
100 %
δ δ
(1)
min 3438
sin cos
tan
2
1
×
−
≈
−
δ δ
(2)
where,
m
is the magnetizing component of exciting
current,
w
is the watt-loss component of exciting current, δ
is the secondary power factor angle and
2S
is the secondary
current.
If the load is intentionally made to be unity power factor,
equations (1) and (2) results in equations (3) and (4).
100
2
−
×
=
(3)
3438
2
×
= (4)
This implies that the measurement of RE and PE in a
transformer test bridge can enable a determination of
and
. If the dimensions of the core are available, magnetizing
ampere turns
!
"
#
= A/m (5)
and watt loss ampere turns
!
"
#
= A/m (6)
can be evaluated, where ! is the mean length of the magnetic
path and "
is the number of secondary turns. If the range of
secondary currents and burdens are chosen appropriately the
instrument transformer test set can be used for characterizing
the performance of the core material. The principal
advantage of this method is that the result is in a form, which
is directly useful for the CT design [1].
682 ISBN 978-963-88410-0-1 © 2009 IMEKO